1. Statement of the Technical Field
The invention is directed to the field of communications. In particular, the invention is directed to systems and methods for increasing security in chaotic communications systems.
2. Description of the Related Art
There are many types of communications systems known in the art, such as multiple access communications systems, low probability of intercept/low probability of detection (LPI/LPD) communications systems and spread spectrum communications systems. Many of these systems depend on equal duration symbols and/or equal energy spreading sequences. These systems have exploitable features that may be used to aid in signal detection and data recovery. Inherent to equal duration, equal symbol energy based waveforms, the signal energy transmitted is stationary for all practical purposes, meaning that the energy transmitted as a function of time is constant. Non-equal energy spreading sequences, such as those used in coherent chaotic waveforms, have also been employed but require significantly more computational power to synchronize. However, communication signals employing equal energy spreading sequences are typically robust against interferers.
Regardless of the type of communications system being used, one common issue in communications systems is to transmit data securely by increasing the computational complexity required for an unintended receiver to decode. Conventional spread spectrum communications systems provide a moderate level of security by virtue of a pseudorandom spreading sequence, especially when the channel encoded waveform is spread using a robust PN sequence. However, if enough information and computation resources are available to an observer, it is possible that the information carried in signal can be discerned. Communication systems employing spreading sequences with a low level of cyclostationary features provide additional security by making the spreading sequence more difficult to synchronize and track without a priori knowledge of the sequence characteristics. Nevertheless, the symbol durations in both types of communications systems are typically chosen based upon a simple deterministic process (e.g. constant), making it easier for an adversary to track/process the data if the spreading sequence is deciphered. Accordingly, in some high security applications, such as military or government communications systems, a higher level of security is desired above that provided by spread spectrum secure communications systems using robust yet cyclostationary spreading functions for generating spread spectrum signals.